The demand for electronics which can be used in challenging environments like for example under the engine hood or in rugged industrial applications is steadily rising. As a result the need for new materials and more efficient power components is also growing. High-power applications involving high temperatures can lead to potentially serious heat problems if components like power transistors (for example metal oxide semiconductor field effect transistors (MOSFETs)), capacitors, resistors or integrated circuits are exposed to such rugged conditions for a long time.
Design technologies for improving heat distribution and integration of new cooling element materials have been proposed to improve thermal management. In addition secondary protective measures have been proposed to stop thermal instability which was caused by the failure of power components or by corrosion-induced heating.
Frequently the use of thermal cutouts or a thermal circuit breaker or a thermoswitch is proposed. Such components afford extensive and specific temperature activation features both in alternating current and also in direct current applications. However they place high demands on the circuit board mounting. As exclusively components for surface mounting are used in relation to more and more circuit boards a component with a through-hole can entail special mounting methods and additional costs and complexity.
In the harsh environment of motor vehicles power semiconductors like for example power MOSFETs are regularly exposed to extreme temperature fluctuations and an extreme thermomechanical stress. Intermittent short-circuits, cold operational environments, arcs and inductive loads and multiple short-circuits can lead to fatigue phenomena on the component in the passage of time so that the component fails in the open, short-circuited or resistive mode.
Although power semiconductors like MOSFETs are becoming increasingly more robust, they are susceptible to failures which can occur very quickly if their nominal values are exceeded. Thus for example when the maximum operating voltage is exceeded an avalanche breakdown occurs. If the energy liberated is above the nominal avalanche energy the components can suffer damage and fail. Power semiconductors in the motor vehicle field are demonstrably more susceptible to fatigue and failure than components which are installed in less demanding situations of use. After five years of use the difference in the failure rates can be greater than a factor of ten. Although such a component can withstand initial tests it has been proved that under certain conditions weak points randomly distributed therein can cause a failure during use.
Even in situations in which MOSFETs operate within the specified operating conditions randomly distributed and unpredictable resistive short-circuits are observed, with different resistance values. The failure in the resistive mode is particularly disturbing, not only for the MOSFETs but also for the circuit boards. A figure of 10 W can already cause a local hotspot of more than +180° C., which is markedly above the typical temperature limit of +135° C. for a circuit board. The epoxy structure thereof is damaged as a result and a thermal event occurs.
Cooling fan modules (CFMs) are essential elements in air conditioning installations and engine cooling systems of vehicles. They cool the engine down and prevent potential overheating under certain conditions, for example in hot weather and when negotiating steep gradients. CFMs are generally integrated under the engine hood and are exposed to more extreme temperature fluctuations, in comparison with modules in the passenger cabin. That thermal loading can accelerate fatigue of the power MOSFET and can lead to premature failure.
There is therefore a need for improved thermal protection of power semiconductors.